With an increase in the recording density of magnetic disks, the distance between the magnetic disk serving as a recording medium and the head for use in recording of information or playback has become almost nil close to contact therebetween. The magnetic disk is provided over the surface thereof with a carbon protective film or lubricant film for the purpose of diminishing abrasion due to the contact or sliding of the head thereon or preventing contamination of the disk surface.
The carbon protective film is produced generally by the sputtering process or CVD process. Since the disk surface is protected with the two films, i.e., the carbon protective film and the lubricant film thereover, the interaction between the carbon protective film and the lubricant is important.
The lubricants generally in use are perfluoropolyethers having functional groups. Examples of functional groups are hydroxyl, amino and cyclophosphazene group. More specific examples of lubricants include Fomblin ZTETRAOL manufactured by Solvay Solexis Inc. and having hydroxyl, and PHOSFAROL A20H, product of MORESCO Corporation and having both hydroxyl and cyclophosphazene groups.
Particularly, lubricants having a phosphazene group are materials having high resistance to decomposition and known as materials for giving high durability to magnetic disks (for example, Patent Literature 1, 2). However, with reference to the lubricant of Patent Literature 1, claim 1 therein defines a compound wherein n is an integer of 1 to 5, whereas the preparation process of the patent concerned affords only a mixture which is not satisfactory in lubricating performance. With the lubricant of Patent Literature 2, a cyclophosphazene ring has a fluoropolyether main chain wherein the number of substituent is 1, and the lubricant is low in bonded ratio as listed in Table 1 given later.
Patent Literature 1: JP2000-260017A
Patent Literature 2: JP2004-352999A
Under the condition wherein the head is at such a distance from the disk that it is almost in contact therewith, the use of the lubricant having a phosphazene group and highly resistant to decomposition involves the problem that the head needs to be prevented from moving into contact with the disk. Heretofore proposed as means for dissolving this problem are techniques for causing the lubricant to adsorb to the carbon protective film more effectively by incorporating at least two hydroxyl groups or like polar groups into the molecule (for example, Patent Literature 3, 4). However, introduction of an increased number of hydroxyl or like polar groups results in enhanced interaction with the carbon protective film to thereby conversely impair the flowability of the lubricant. In the case of serious impairment of flowability, insufficient lubricity will result under the lubricating condition wherein the head is nearly in contact with the disk, possibly reducing the durability of the magnetic disk.
Patent Literature 3: WO2006009057
Patent Literature 4: WO2007043450
An object of the present invention is to provide a fluoropolyether lubricant which has a cyclophosphazene group and hydroxyl in the molecule and wherein the fluoropolyether is given a narrowed molecular weight distribution so as to make the lubricant resemble single compounds and thereby render the lubricant satisfactory in both adsorption to the disk surface and flowability thereon.